Fix cart: start session in functions.php so API endpoints persist cart

All api/*.php files include functions.php but none called session_start(),
so $_SESSION writes were lost after each request. Cart appeared to work
(API returned cart_count:1) but nothing was ever saved.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/functions.php

@@ -6,6 +6,10 @@
 require_once __DIR__ . '/../config/config.php';
 require_once __DIR__ . '/db.php';
 
+if (session_status() === PHP_SESSION_NONE) {
+    session_start();
+}
+
 /**
  * Generate a unique ID with prefix
  */